Exploration for and production of hydrocarbons in subterranean reservoirs is a highly technical and expensive undertaking. Even in areas with existing wells that can provide direct data about the subsurface, understanding the reservoir properties away from the well locations is difficult. Additionally, the data from the well locations may not be completely representative of the entire subsurface volume due to the natural variation of the rock.
Understanding the areal distribution of porosity in subsurface reservoirs is important for calculating potential hydrocarbon content and production expectations. Although methods exist to estimate porosity based on seismic impedance, these methods perform poorly in so-called unconventional reservoirs, which have low porosity due to the type of rock (e.g. shale, carbonates).
Seismic exploration involves surveying subterranean geological media for hydrocarbon deposits. A survey typically involves deploying seismic sources and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological medium creating pressure changes and vibrations. Variations in physical properties of the geological medium give rise to changes in certain properties of the seismic waves, such as their direction of propagation and other properties.
Portions of the seismic waves reach the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy one type of sensor or both. In response to the detected seismic waves, the sensors generate corresponding electrical signals, known as traces, and record them in storage media as seismic data. Seismic data will include a plurality of “shots” (individual instances of the seismic source being activated), each of which are associated with a plurality of traces recorded at the plurality of sensors. The amplitude of the seismic response varies with offset (the distance from the source to the sensors) (AVO) and reflection angle (AVA). Conventional methods for predicting subsurface properties based on AVA, such as U.S. Pat. No. 7,869,955, require well log data either from numerous drilled wells or pseudo-wells. In many areas there are few drilled wells and pseudo-wells may not be representative of the entire subsurface volume of interest, particularly in low-porosity reservoirs.
The ability to estimate porosity in the subsurface and, by extension, the location of rock and fluid property changes in the subsurface is crucial to our ability to make the most appropriate choices for purchasing materials, operating safely, and successfully completing projects. Project cost is dependent upon accurate prediction of the physical geologic properties within the Earth. Decisions include but are not limited to, budgetary planning, obtaining mineral and lease rights, signing well commitments, permitting rig locations, designing well paths and drilling strategy, preventing subsurface integrity issues by planning proper casing and cementation strategies, and selecting and purchasing appropriate completion and production equipment.
There exists a need for improved porosity estimation in low-porosity hydrocarbon reservoirs, allowing more accurate porosity mapping to facilitate exploration for and production of hydrocarbons.